1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus and imaging system.
2. Description of the Related Art
A photoelectric conversion apparatus of a CMOS image sensor includes a pixel including a photoelectric conversion element, a read circuit adapted to read a signal resulting from photoelectric conversion performed by the pixel. Also, depending on a reading system and added functions, the photoelectric conversion apparatus includes a sensitivity switching circuit, a sampling and holding circuit, a scanning circuit, and the like.
FIG. 2 of Japanese Patent Application Laid-Open No. 2011-139427 discloses a circuit diagram showing configurations of a unit pixel and transfer unit of a photoelectric conversion apparatus. The photoelectric conversion apparatus includes a MOS transistor and capacitor for sensitivity switching. Description will be given of a case in which the capacitor is electrically cut off from an input terminal with the MOS transistor turned off. There is an energy band gap between a valence band and conduction band. At a pn junction between a source region and sub-gate region of the MOS transistor, energy band gap width in a direction from the source region toward the region under the gate is smaller than when the MOS transistor is on. Consequently, electrons tend to get across the energy band gap in a forward direction, making it easier for a tunneling current to occur. The tunneling current becomes a dark current of the photoelectric conversion apparatus. The tunneling current produced when the MOS transistor is off is accumulated on the input terminal or capacitor to become a dark current. The dark current, which is a noise, has a problem of reducing a signal-to-noise ratio of the photoelectric conversion apparatus.